As semiconductor devices have become more highly integrated in recent years, circuit interconnections have become finer and devices to be integrated have been multilayer devices. Therefore, it is necessary to planarize a surface of a semiconductor wafer. It has been customary to remove surface irregularities from the surface of the semiconductor wafer by a chemical mechanical polishing (CMP) process so as to planarize the surface of the semiconductor wafer.
According to the chemical mechanical polishing process, after the semiconductor wafer has been polished for a certain period of time, the polishing needs to be finished at a desired position on the semiconductor wafer. For example, it may be desirable to leave an insulating layer such as SiO2 over a metal interconnection of Cu or Al (such an insulating layer is referred to as an interlayer film because a metal layer will be formed on the insulating layer in a subsequent process). If the semiconductor wafer is polished more than required, then a lower metal film is exposed on the surface. Therefore, the polishing process needs to be finished in order to leave a predetermined thickness of the interlayer film.
According to another process, a predetermined pattern of interconnection grooves is formed in a surface of a semiconductor wafer. After the interconnection grooves are filled up with Cu (copper) or Cu alloy, unnecessary portions are removed from the surface of the semiconductor wafer by the chemical mechanical polishing (CMP) process. When the Cu layer is polished by the CMP process, it is necessary to selectively remove the Cu layer from the semiconductor wafer, while leaving only the Cu layer formed in the interconnection grooves. Specifically, the Cu layer needs to be removed to expose an insulating film of SiO2 or the like in areas other than the interconnection grooves.
In this case, if the Cu layer in the interconnection grooves is excessively polished off together with the insulating layer, then the circuit resistance will be increased, and the entire semiconductor wafer will have to be discarded, resulting in a large loss. Conversely, if the Cu layer is polished insufficiently and remains on the insulating layer, then circuits will not be separated well, thus causing short-circuits. As a result, the Cu layer needs to be polished again, resulting in an increased manufacturing cost.
Thus, there has been known a polishing state monitoring apparatus for measuring the intensity of reflected light with an optical sensor and detecting an end point of the CMP process based on the measured intensity of reflected light. Specifically, the polishing state monitoring apparatus has an optical sensor comprising a light-emitting element and a light-detecting element, and light is applied from the optical sensor to a surface, being polished, of a semiconductor wafer. A change of the reflectance of light in the surface, being polished, of the semiconductor wafer is detected to detect an end point of the CMP process.
The following processes for measuring optical characteristics in the CMP process are known in the art:
(1) Light from a monochromatic light source such as a semiconductor laser, a light-emitting diode (LED), or the like is applied to the surface, being polished, of the semiconductor wafer and a change in the intensity of reflected light is detected.
(2) White light is applied to the surface, being polished, of the semiconductor wafer, and the spectral reflectance thereof is compared with a pre-recorded spectral reflectance at a polishing end point.
In this specification, the spectral reflectance is defined as a term including “spectral reflectance” and “spectral specific reflectance”. The spectral reflectance is defined as “ratio of energy of reflected light to energy of incident light”. The spectral specific reflectance is defined as “ratio of energy of reflected light from an object to be monitored to energy of reflected light from a reference (for example, bare silicon wafer)”.
Recently, there has been developed a polishing state monitoring apparatus for estimating an initial film thickness of a wafer, applying a laser beam to the wafer, and approximating time variation of the measured value of the intensity of reflected light from the wafer with a sine-wave model function to calculate the film thickness.
In the conventional polishing state monitoring apparatus, however, the positions of sampling points on the surface, being polished, of the semiconductor wafer are not controlled, and the sampling points are changed depending on the initial angular position, the rotational acceleration, and steady rotational speed of the polishing table, and the time to start the sampling process. Therefore, characteristic values such as a film thickness at desired positions on the wafer surface, for example, a central line on the wafer or peripheral portion on the wafer cannot be measured. Particularly, if the sampling period is long, then it is difficult to estimate a remaining film profile.
In the above-mentioned polishing state monitoring apparatus which measures the film thickness using the model function, the film thickness is calculated based on an expected initial film thickness and time variation of the measured value of a reflection intensity. Consequently, if the polishing rate varies during the polishing process, or if it is difficult to estimate an initial film thickness, or if an initial film thickness is small, then an accurate model function cannot be determined, thus making it difficult to measure a film thickness.
If the sampling period is long and one sampling point (sampling region) is in a wide range over the surface of the wafer, then various film thicknesses depending on different patterns and removal quantities are measured at one time. Consequently, an accurate model function cannot be determined, and hence it is difficult to measure a film thickness.
In the CMP process, the intensity of reflected light from the surface, being polished, of the wafer varies due to the effect of a slurry (polishing liquid), air bubbles, or mechanical vibrations. Specifically, if a monochromatic light source is used, then fluctuations of the intensity of reflected light directly cause measurement errors. If white light is used, then fluctuations of the spectral reflectance also directly cause errors, thus lowering the accuracy of an end point detection.